Air forks - air in one leg only?

Hi, went riding the other day and before it my brother in law was setting up his SID, and i noticed that he had to pump air into the top of the crown on BOTH sides. But on my Revelation and what i notice on other forks, air adjustments are made only on one location (except for RS dual air but thats a different concept and the adjustments are still on one leg of the fork).

I always thought that the air pumped in gets into the other leg as well, but now i'm starting to think that one leg of the fork is responsible for the air pressure, and the other is responsible for the damping and other functions.

If thats the case, wouldn't it skew the suspension performance if air is only in one of the legs? Or does the air actually spread into both legs? Can anyone shed some light on this?

The structure is stiff enough that one side does the air spring and the other side does the damping.

The Specialized S120 goes even further with the damping in the same leg as the air spring and the other leg is essentially "empty" and only exists to deliver steering inputs to the axle.

Think about the Cannondale Lefty...

If he had to put...

air in both sides of his SID, it's a pretty old SID. The one sided spring concept has been around for a long time and has vastly improved fork performance. The problem with a dual spring fork is space for the damper. When travel was relatively short it wasn't so much of an issue. Most dampers were simply a cartridge that sat below the spring stack in one leg. However as travel got longer and riding styles more agressive the damper cartridge showed it's weakness. Low oil volume and inability to disipate heat caused them to fail at a very high rate. Also it was difficult to add compression damping to this style of fork. The initial solution was an open oil bath style of damping system ala Marzocchi and some others. This worked well, but the open bath oriface type damper had it's draw backs, under a hard impact it was prone to hydraulic spiking under hard fast compression. Another disadvantage of the open bath system is the same oil that lubricates the bushings etc. also serves as the damper oil. Any crud that got into the lowers or bits that wore off the bushings could contaminate the damper and damage the seals. With a single sided spring you have the entire oposite leg to contain the damper. The stanchion is actually the damper body in many designs. The advantages are much higher oil volume, better cooling, simpler to add compression damping to the unit, larger size heavier duty components, larger more effective seals, 0 external contamination, etc.

The signle sided spring has 0 affect on the performance, or longevity of the fork. Air is not intoduced into the other leg, there's no reason to, and no room for it anyway, it's all full of the damping system. So when you fill your one air spring on your fork that's the only place the air goes that one leg.

Modern single sprung forks, whether air or coil perform VASTLY better than the older dual sprung forks of years ago. Take it from someone that's been riding long enough to have ridden allot of the older dual sprung, cartridge damped forks of old. The single spring type fork doesn't skew the performance of the spring, it's lighter, requires much less maintenance, and is vastly more durable and reliable, and the dampers work much better. It isn't just the single side spring that has improved fork performance over the older dual sprung forks. There are plenty of other advances in suspension technology that make up the difference as well. But the single sided spring is a significant chunk of it.

Good Dirt

The Sid "Race" up until the 2008 model year was equipped with the Pure damper. It used air pressure in the right leg for low speed compression damping. This might be the reason air was added to the top of both legs.

Some damping systems require air pressure to perform properly so it isn't uncommon to see air valves on both fork legs even if one side isn't a spring. Manitou SPV and the Marzocchi TST system are two others that you would see an air adjustment on the damper leg.